Field of the Invention
Embodiments of the invention relate to methods of fabricating semiconductor devices and methods of fabricating a system of semiconductor devices.
Related Art
In recent years, thickness reduction of the semiconductor wafer of an IGBT (Insulated Gate Bipolar Transistor) is being carried out for enhancing the performance and reducing the cost of the IGBT. For example, for enhancing the performance and reducing the cost of an IGBT, it is necessary to thin the thickness of a semiconductor wafer down to the order of 50 μm to 100 μm or less.
For a fabrication method of a semiconductor device with a reduced device thickness, the following has been proposed. According to the method, a large number of device structures are formed on the top surface of a semiconductor wafer before carrying out processing such as back grinding or silicon etching of the bottom surface of the semiconductor wafer until the thickness of the semiconductor wafer is reduced to become a specified thickness. Then, dicing is carried out for each of the device structures formed on the top surface of the semiconductor wafer to cut the semiconductor wafer for being separated into individual semiconductor chips each having the device structure.
For another fabricating method of a semiconductor device with a reduced device thickness, the following has been proposed. According to the method, after device structures are formed on the top surface of a semiconductor wafer, grooves are formed on dicing lines on the top surface of a semiconductor wafer each with a depth larger than the specified thickness of a completed semiconductor device, for example. Then, processing such as back grinding or silicon etching of the bottom surface of the semiconductor wafer is carried out until the thickness of the semiconductor wafer is reduced to become a specified thickness, by which the grooves formed on the top surface of a semiconductor wafer make the semiconductor wafer separated into individual semiconductor chips.
As another method of fabricating a semiconductor device with a reduced device thickness, the following has been proposed. According to the method, after device structures are formed on the top surface of a semiconductor wafer, processing such as back grinding or silicon etching of the bottom surface of the semiconductor wafer is carried out to thin only the middle section of the semiconductor wafer in the range with a diameter smaller than the diameter of the semiconductor wafer to leave the peripheral section (hereinafter referred to as a rib section) of the semiconductor wafer unprocessed. Then, dicing of the semiconductor wafer is carried out with the rib section being left unprocessed or after the rib section is removed to separate the semiconductor wafer into individual semiconductor chips.
In carrying out dicing of the semiconductor wafer in this way, for avoiding a problem of scattering semiconductor chips separated by dicing, a method is known in that dicing is carried out with an adhesive sheet such as a dicing sheet stuck onto the bottom surface of a semiconductor wafer, for example. After the dicing is carried out with the adhesive sheet stuck onto the bottom surface of a semiconductor wafer, semiconductor chips being stuck to the adhesive sheet to be fixed thereto are supplied to a pick up system for being picked up from the adhesive sheet to be separated into individual semiconductor chips.
For a method of picking up semiconductor chips from an adhesive sheet, the following has been proposed. According to the method, each of the semiconductor chips is pushed upward by a tool such as a needle, for example, from the side of the bottom surface, onto which the adhesive sheet is made to stick, to reduce the contact area of the semiconductor chip with the adhesive sheet. Then, by a tool such as a collet, suction is applied to the semiconductor chip pushed upward by the needle to pick up the semiconductor chip from the adhesive sheet. However, when such picking up method is applied to a thinned semiconductor chip, the needle pushing up the semiconductor chip might cause flaws on the bottom surface of the semiconductor chip or breakage of the semiconductor chip.
For solving such a problem, as a method of reducing the contact area of the semiconductor chip and the adhesive sheet without carrying out pushing by a needle, the following method is proposed. According to the method, a fixed jig is used which includes a jig base and an adhesion layer. The jig base has a plurality of projections and a side wall on one side. The adhesion layer is layered on the surface of the jig base having the projections and is bonded on the top surface of the side wall. On the surface of the jig base having the projections, zoned spaces are formed by the adhesion layer, the projections and the side wall. Each of the zoned spaces is connected to a vacuum source by a through hole. Through the through holes, suction is applied to the air in each of the zoned spaces to deform the adhesion layer. Along with this, from the top surface side of the chip, a collet applies suction to the chip to pick up the chip from the adhesion layer (see, e.g., Japanese patent publication no. J P-A-2008-103493).
As another method, the following method has been proposed. According to the method, on a stage having a plurality of suction grooves and a plurality of projections, chip-like pieces are mounted with adhesive holding sheets, bonded onto the bottom surfaces of the pieces, made to be in contact with the stage. A plurality of the suction grooves are distributed so as to cover regions facing the chip-like pieces and a plurality of the projections are made to be positioned so as to partially face each of the chip-like pieces at least at two positions between the suction grooves. By applying a negative pressure to the suction grooves, the holding sheets are removed from the chip-like pieces while being made deformed so as to be stuck along the projections. At this time, the negative pressure applied to the suction grooves facing the peripheral section of a chip-like piece is made to be stronger so that the holding sheet is removed from the peripheral section of the chip-shaped piece (see, e.g., Japanese patent publication no. JP-A-11-54594).
As yet another method, the following method has been proposed. According to the method, on the surface of an attraction piece on which surface the bottom surface side of a semiconductor chip is mounted with a dicing tape put in between, an attraction face and a side wall are provided. The attraction face has a plurality of projections of the same height provided so as to stand perpendicularly. Each of the projections has a top surface that is concave upward and approximately hemispherical. The side wall is provided over the whole periphery of the attraction face with a width of 0.4 mm or less and a height being equal to the height of the projection or having a difference less than 1 mm from the height of the projection. Moreover, a suction hole is provided in at least one of a bottom between adjacent projections, a side surface of the projection, or each of them. With each of the suction holes, the dicing tape is made attracted to the projection. Along with this, on the top surface side of the semiconductor chip, a collet picks up the semiconductor chip. When suction is applied to the dicing tape with the use of the suction holes, the suction is applied so that the dicing tape is made to be brought slightly downward between the side wall and the attraction face of the attraction piece within the elastic limit of the dicing tape (see, e.g., Japanese patent publication no. JP-A-2010-123750).
In addition, as a pick up system for preventing a semiconductor chip from such damage as being scratched, the following system is proposed. According to the system, a wafer stage includes a plurality of projections, a plurality of suction grooves and a vacuum unit. A plurality of the projections hold the bottom surface of each of IC chips at their tops through an adhesive sheet. A plurality of the suction grooves are formed in some of bottom sections each formed between adjacent projections of a plurality of the projections. The vacuum unit is connected to the suction groves through connection tubes and generates a force of attraction at the suction grooves to thereby separate the adhesive sheet from the IC chip for making the adhesive sheet attracted to the bottom sections each formed between the adjacent projections (see, e.g., Japanese patent publication no. JP-A-5-335405).
With each of the methods of the related art disclosed in the foregoing, however, the following problems arise as shown in FIG. 16. FIG. 16 is a cross-sectional view showing semiconductor chips in a state of being held on a holding stage in a related fabrication system of semiconductor devices. In FIG. 16, a state is shown in which an adhesive sheet 102 securing a plurality of semiconductor chips 101a is deformed so as to be stuck along projections 111a on a holding stage 111. When the adhesive sheet 102 is made to be deformed as shown in FIG. 16, the difference in timing at which the adhesive sheet 102 is removed from the same bottom surface of a semiconductor chip 101a-1 brings the semiconductor chip 101a-1 into a state of being inclined to the holding stage 111 in applying suction to the semiconductor chip 101a-1 by a collet.
In the case when the semiconductor chip 101a-1 is brought into the state of being inclined to the holding stage 111, there is the possibility of causing one end portion of the semiconductor chip 101a-1 to contact with an end portion of an adjacent semiconductor chip 101a-2 to make the semiconductor chips 101a-1 and 101a-2 get chipped. Also in the case when the semiconductor chip 101a-1 is brought into the state of being inclined to the holding stage 111, there is the possibility of causing the end portion on the raised side of the semiconductor chip 101a-1 to hit a collet (not shown) to damage the semiconductor chip 101a-1.
In addition, the ease of deformation of the adhesive sheet 102 attracted to the projections 111a differs depending on factors such as that the adhesion of the semiconductor chip 101a to the adhesive sheet 102 differs depending on the surface roughness or the presence or absence of a metal film of the bottom surface of the semiconductor chip 101a and that the adhesion or stiffness of the adhesive sheet 102 varies depending on the kind of the base material and the quality of the material forming the adhesive sheet 102. Thus, it is difficult to control the constant deformation of the adhesive sheet 102 into the form of the projections 111a. Therefore, with the methods of the related art explained in the foregoing, it is impossible to prevent the semiconductor chip 101a-1 from being brought into a state of being made inclined to the holding stage 111.
Thus, there is a need in the art for to provide a fabrication method of semiconductor devices and a fabrication system of semiconductor devices in which a semiconductor chip is brought into a state of being made inclined to a holding stage.